expand

Override this method to specify how this PTransform should be expanded
on the given InputT.

NOTE: This method should not be called directly. Instead apply the
PTransform should be applied to the InputT using the apply
method.

Composite transforms, which are defined in terms of other transforms,
should return the output of one of the composed transforms. Non-composite
transforms, which do not apply any transforms internally, should return
a new unbound output and register evaluators (via backend-specific
registration methods).

populateDisplayData

populateDisplayData(DisplayData.Builder) is invoked by Pipeline runners to collect
display data via DisplayData.from(HasDisplayData). Implementations may call
super.populateDisplayData(builder) in order to register display data in the current
namespace, but should otherwise use subcomponent.populateDisplayData(builder) to use
the namespace of the subcomponent.

By default, does not register any display data. Implementors may override this method
to provide their own display data.